1. Field of the Invention
This invention relates generally to a process for testing semiconductor devices, and relates more particularly to a process for measuring CMOS device output capacitance and output slew rate from switching-induced hot carrier luminescence.
2. Description of the Relevant Art
Measuring switching-induced hot carrier luminescence from semiconductor transistors such as CMOS devices is a passive, noninvasive optical method for monitoring device operation. It is sometimes referred to as Pico-second Imaging Circuit Analysis (PICA). When current flows through a switching CMOS device, the supply voltage accelerates electrons, which in turn causes photon emission in the near infrared region. The light is emitted close to the drain where the electric field is the most intense. Typically, more photons are generated from an nMOSFET device than a pMOSFET device, since carrier mobility is higher in an nMOSFET.
This technique allows monitoring of individual semiconductor devices. The front side of a typical integrated circuit is covered with metallization layers that may obscure the semiconductor devices and the metal interconnections that carry the electrical signals, so that the semiconductor devices and many of the signal-carrying metal lines may not be accessible to external probes for testing. Furthermore, flip-chip packaging orients the top side of an integrated circuit downward in the package, which further complicates access to individual semiconductor devices or signal lines for testing.
The PICA technique, however, overcomes these limitations because the switching-induced hot carrier photons can be detected from the back side of the integrated circuit. Silicon is transparent to light at the near infrared wavelengths of the hot carrier photons, so some of the emitted photons emerge from the back side of the integrated circuit where they can be detected. Thus, the method of detecting intrinsic photon emission from switching devices through the device backside provides superior circuit observability.
The PICA technique uses optics to image the emitted photons onto a photodetector. The switching-induced hot carrier luminescence emits photons only during switching, but not every switching event generates a photon and not every photon is detected. Consequently, the device must be cycled on and off and the photons collected over extended periods, like several minutes. When the detected photons are time-correlated and integrated, timing analysis is possible to pico-second accuracy.
The collection of timing data during device switching in the pico-second realm has been the primary use of this intrinsic luminescence. Switching-induced hot carrier luminescence can be used to measure internal device timing at pico-second accuracy. This is a time-correlated photon counting technique in which a single photon detector is synchronized with device test stimulus. Time-resolved photon measurements were obtained through time integration of photon counting.
What is needed, however, is the ability to measure more than the timing of a device. It would be beneficial to be able to measure other parameters such as output capacitance and output slew rate.
In summary, the present invention is a process for measuring semiconductor device output capacitance and slew rate from switching-induced hot carrier luminescence. CMOS devices are the preferred semiconductor devices to be measured with the present invention.
The present invention includes a process for determining the output capacitive loading of a semiconductor device by measuring the peak switching-induced hot carrier luminescence and comparing it to previously correlated capacitance data. In particular, the process includes preliminary steps of analyzing a process test device to determine its node capacitances, applying a driving signal to the process test device and detecting its peak switching-induced hot carrier luminescence, and correlating the node capacitances and the detected peak luminescence to establish a correlation between output capacitive loading and peak luminescence. The process further includes applying a driving signal to the semiconductor device under test and detecting its peak switching-induced hot carrier luminescence, and determining the output capacitive loading of the semiconductor device by using the correlation between output capacitive loading and peak luminescence. The process test device is a test circuit that is made by the same or a similar process as the semiconductor device and that has known or measurable parasitics so that its RC characteristics can be extracted and used to establish the correlation.
The present invention also includes a process for determining the output slew rate of a semiconductor device by measuring the switching-induced hot carrier luminescence as a function of time, calculating a standard deviation of the luminescence data, and comparing it to previously correlated output slew rate data. In particular, the process includes preliminary steps of analyzing a process test device to determine output slew rates thereof, applying a driving signal to the process test device and detecting its switching-induced hot carrier luminescence as a function of time, fitting the detected luminescence of the process test device to a Gaussian curve and determining its standard deviation, and correlating the output slew rates and the standard deviation of the luminescence of the process test device to establish a correlation between the two. The process further includes applying a driving signal to the semiconductor device under test and detecting its switching-induced hot carrier luminescence as a function of time, fitting the detected luminescence of the semiconductor device to a Gaussian curve and determining a standard deviation thereof, and determining the output slew rate of the semiconductor device by using the correlation between output slew rate and the standard deviation of the detected luminescence.
In summary, the peak of a switching-induced hot carrier luminescence pulse directly relates to the driving capacitance and the standard deviation of a pulse relates to the rate of change of output voltage or slew rate.
The features and advantages described in the specification are not all inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter.